In an effort to more fully explore the

In an effort to more fully explore the structure–activity relationships of the ALLINIs and potentially attenuate resistance to known mutants, the central scaffold of these compounds was identified as a potential site for structural manipulation. Specifically, we wished to examine whether a scaffold hopping approach could be applied to this class through transfer of the key binding elements from a six-membered to a five-membered heterocyclic ring and whether this new ring system could effectively orient these groups in the LEDGF/p75 binding site. In analogy to the quinoline core found in many ALLINIs, the indole presents a similar bioisosteric benzannular nitrogen-containing framework, despite significant differences in electron density, relative geometry, and H-bonding capabilities. Synthetically, however, there are several potential benefits to employing an indole in the synthesis of ALLINI analogues, namely, the wide array of existing methods for the preparation of the core and the relative ease of functionalization around the periphery of this scaffold, both of which could be expected to further the development of new ALLINI analogues. With this in mind, a computational docking model of an indole based analogue with HIV-1 IN was examined using AutoDock 4.0 (). This model predicted that indole-based analogues would maintain a binding mode similar to that of the prototypical quinoline-based ALLINI, BI-1001. Of note, as predicted in our preliminary structural analysis, the docking model indicates that the geometry of the smaller five-membered ring of the indole imparts a modest effect on the relative positioning of the core in the binding pocket. While the key acetic chloroquine phosphate and aryl substituents are predicted to overlay well in both the quinoline and indole systems, a slight ‘tilt’ of the indole aromatic ring away from the A128 residue relative to the quinoline ring system is observed, suggesting that the indole analogues may not interact with that site or be affected by the A128T mutation which confers marked resistance to quinoline-based ALLINI analogues. Encouraged by these results, we pursued the synthesis of a series of indole analogues to experimentally test these predictions. Key considerations in the development of these analogues were the efficiency of the route and the ability to easily introduce structural variation at the C3 substituent. Methods previously utilized to introduce functionality at this position in both the quinoline and pyridine systems have relied upon coupling reactions, typically a Suzuki coupling, to introduce the aryl substituents. A new strategy for the functionalization of the indole core could utilize alternative reactivity, ultimately facilitating the incorporation of other functional groups at that position. With this in mind, the inexpensive, commercially available reagent isatin was employed as a starting material for the synthesis () and was quickly transformed using well-established methodology to the C3-substituted indole via nucleophilic addition of Grignard or aryllithium reagents and subsequent hydride reduction. Alkylation of the indole nitrogen was then carried out using either iodomethane (compounds –) or benzyl bromide (compound ). Installation of the critical acetic acid side chain was then accomplished through acylation of the electron rich indole ring system with oxalyl chloride followed by the addition of methanol to give ester . Selective reduction of the ketone with sodium borohydride cleanly provided alcohol . Alkylation using the resin bound acid, Amberlyst 15, successfully generated the desired -butyl ether, although other conditions established for the introduction of the -butyl ether functionality including the use of either -BuOAc/perchloric acid or -butylacetimidate have also been successfully applied to the indole system., Finally, saponification of the methyl ester furnished the sodium salt of the indole analogues –. Acidification of the salts to pH=4 provided the acids, but further lowering of the pH resulted in degradation of the compounds, presumably due to the potential reactivity of the indole ring in combination with the instability of the -butyl group under these conditions. Due to the sensitivity of the indole system and the low yields obtained in the saponification and subsequent acidification, in several cases the acids were not protonated and the isolated carboxylates (sodium salts) were directly submitted for biological evaluation. The acidification was not considered critical in this case due to the use of neutral buffer conditions for the bioassay in which the molecule exists as the carboxylate anion. In total, five indoles (–) were synthesized by employing this seven step sequence from isatin.